Controllable preparation and properties of gel-casting porous mullite ceramics via lignin fiber assisted pore formation method

The preparation of high-strength porous mullite via gel-casting has emerged as a hot topic, but balancing high strength and high porosity remains challenging. Herein, we successfully prepared porous mullite with uniform pore size and high strength using natural lignin fibers as the pore-forming agent via gel-casting. Moreover, we investigated the properties of mullite slurries, as well as green bodies and sintered ceramics in details. The strong water transport capacity of the lignin fibers enabled the homogeneous dispersion of gel system and improved the flowability of slurry. The flexural strength of green body was significantly increased to 16.07 MPa by optimizing the lignin fiber content. Porous mullite ceramics sintered at 1400 °C exhibited a homogeneous microstructure with average pore size of 5.62 μm. Furthermore, mullite ceramics had a porosity of 64.73% at a total solid loading of 55 wt% and a lignin fiber content of 10 wt%, whereas the compressive strength remains high at 11.32 MPa. Permeability tests confirmed the presence of interconnected pores, its coefficients k1 and k2 increases with the total porosity of ceramics. This work provides new insights into designing the preparation of porous mullite ceramics with high performance.